1. Field of the Invention
The field of the invention is catalytic cracking of heavy hydrocarbon feeds to lighter products.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree.-600.degree. C., usually 460.degree.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 600.degree.-900.degree. C., usually 600.degree.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds.
The product distribution from modern FCC units is very good, in that the amount and octane number of the gasoline product is very satisfactory, and the light ends are readily upgraded in sulfuric or HF alkylation units to produce high quality alkylate. Unfortunately, refiners are finding it more and more difficult to make enough gasoline of sufficient octane while also meeting new specifications in regards to the amount of oxygenates, aromatics and benzene in the fuel. Reduced limits on RVP (Reid Vapor Pressure) and gasoline end point reduce the amount of butanes that can be added, further exacerbating the problem.
We wanted to develop a way to squeeze more gasoline and distillate out of FCC processing, and also to change the quality and quantity of the light ends made by the FCC process. We wanted more iso- compounds, which have a higher octane number and which are also more reactive in other processing units, e.g., etherification and alkylation. We wanted to produce a higher quality distillate product (LCO) containing less aromatics. We wanted to reduce the FCC process light gas make, to minimize the capital and operating expense of the FCC light gas processing equipment. We wanted to reduce overall process coke make and catalyst circulation rates which bottleneck many existing FCC unit operations. We also wanted to provide way to reduce the FCC regenerator emissions, including particulate and CO/CO2 emissions.
The way conventional FCC processes operate, we were severely limited. The trend in modern FCC units is to higher riser temperatures, and shorter contact times in the riser reactor and heavier feeds and extremely high conversions. These conditions increase olefin yields, but the higher temperatures associated with such operation reduce the production of iso-olefins and iso-paraffins, while increasing the undesired production of coke and light ends.
We realized that conventional FCC processing had generally been optimized in regards to conversion of "the bottom of the barrel", with constant pushing of the unit to tolerate ever heavier feedstocks. This approach yielded considerable success, and enabled many FCC units to process feeds containing 5 to 10 wt % of non-distillable or residual feedstock. While processing of heavy feeds usually produced a gasoline fraction of reasonably high octane content, and produced a reasonably large amount of olefins, there was relatively low production of iso- compounds. Thus FCC units were evolving to process heavier feeds, of worse quality, but in so doing were also making it harder to efficiently produce clean fuels for gasoline engines.
We decided to take a different approach. We realized that two stage processing of FCC feeds was necessary to get a breakthrough in iso-component yields, but an entirely different kind of two stage processing than had heretofore been used.
Two stages processing of hydrocarbon feeds in FCC is common for heavy feeds, those containing large amounts of non-distillables and metals. Several processes have been developed in which a fluidized first stage of processing removes much of the metals and non-distillables from the feed, to produced a demetallized product. The demetallized product is then processed in a more conventional FCC unit. The first stage of processing uses a low activity, cheap contact material, which transfers heat to the feed and provides an abundance of surface area for deposition of metals and Conradson Carbon Residue (CCR). The first stage of processing is a relatively low severity thermal reaction, something like severe visbreaking (though no liquid phase is maintained ) or a mild fluid coking process. Although the low severity might seem beneficial, the conversions achieved in the first stage (thermal) are so low) and that low yields of iso-components are achieved.
A better approach, at least as far as maximizing the yield of gasoline from an FCC unit, was multi-stage processing of the feed. Several multi-stage processes are known which increase the quantity of gasoline produced, without seriously reducing the iso-component production of the unit.
One of the most interesting two stage catalytic cracking approach was developed by Shell, in the course of which they also developed riser cracking of fresh feed. The first stage was a moderate severity riser reactor, followed by a fractionator, followed by a conventional (at the time) dense bed cracking reactor. The first stage operated at a relatively short contact time, and at a relatively high temperature. This development in 1956 was ahead of its time, about a decade before the development and use of zeolite cracking catalyst. These developments were reported by Heldman, J. D., et al, Proc. API (III) Vol. 36, 1956, pp. 258-264.
This staged approach to maximize gasoline yields, was taken to an extreme degree by Mobil researchers Farber, Payne and Sailor who, in 1965, used multiple stages of cracking, over a moving bed (bead) zeolite containing catalyst, to get what might be considered the ultimate yields of gasoline in catalytic cracking. The cumulative advantage in gasoline yields for the low conversion per pass process, with intermediate gasoline removal, was 24% at 80% conversion. This work was published in FIG. 38 "Ultimate Yield in Cracking Over Zeolite, of Fluid Catalytic Cracking Report, p. 33, O & GJ Jan. 8, 1990, by Avidan et al.
Unfortunately, such multi-stage contacting causes lower light olefin yields. The iso to normal ratio of light olefins would be good, but the total yield of them declines drastically. The process converted more of the feed into gasoline boiling range materials, and would also make less light olefins. Such an approach would make more gasoline, but would require tremendous capital and operating expense for multi-stage cracking of FCC feed, and starve most alkylation units for light olefins needed to make alkylate. There would also be some yield loss, because each pass through an FCC reactor means some product is lost due to poor stripping.
We discovered that by using a two stage unit, with a profoundly different catalyst in the first stage we could break free of the constraints of current FCC units. With our two stage unit, we could greatly enhance the yield of desired iso-compounds, and minimize coke, light gas make and aromatics yields, while retaining the ability to process heavy feeds.
We realized that use of a shape selective first stage catalyst, preferably one with activity too high to permit its use in a conventional cracking unit, preferably at low severity conditions, provided the key to achieving the above objectives.
We preferred to send through the first stage unit a relatively light charge stock, even including such hydrocarbons as heavy naphthas, and avoid non-distillable materials. Thus we preferably use an unusually light feed, and an unusually shape selective, and preferably very active, catalyst in the first stage. Resid feeds, if any, are preferably sent only to the second stage which is a conventional FCC unit.
In contrast, the two stage resid cracking processes used a relatively inert first stage "catalyst". The Shell two-stage cracking process used a high temperature, short contact time riser cracking first stage, with a conventional cracking catalyst, processing all of the feed through both reactors. Our process uses a shape selective first stage catalyst in a two stage process to achieve some unusual results.